1. Field of the Invention
This invention relates generally to yeast autolysis processes and, more particularly, to a method for increasing the efficiency of yeast autolysis by the addition of certain exogeneous enzymes. The resulting yeast autolyzate is particularly effective as a growth stimulating nutrient for the preparation of dairy starter cultures for cheese making or other milk fermentations.
2. Description of the Prior Art
Autolyzed yeast and yeast autolyzate (also called autolyzed yeast extract), two different products from the yeast autolysis, have long been used as seasoning ingredients in the food industry. They are also important nutrient sources for various fermentation processes. Autolysis occurs when endogenous enzymes, mainly proteases and ribonucleases, digest the intracellular high molecular weight components of the yeast cells. The process can be induced by heating yeast to a temperature (about 50.degree. C.) where the cell is killed, but the enzyme systems are active. It can be stimulated also by adding plasmolysing or liquefying agents such as salt and organic solvents. During autolysis, macromolecules are hydrolyzed and the soluble degradation products of small molecular size, such as peptides, amino acids, nucleotides, etc., diffuse out from the cells. It appears that the solubilization of organic nitrogenous material is the main aspect of autolysis.
Yeast autolysis can be carried out at temperatures ranging from about 30.degree.-60.degree. C. In general the process is extremely slow and at low temperatures may require 3-7 days for completion. At 50.degree.-55.degree. C., total digestion can be completed within 3 days, but in practice, shorter processing times are employed while sacrificing autolyzate yield. At the lower temperatures of 30.degree.-40.degree. C. the addition of antiseptics to suppress putrefaction is necessary, although antiseptics can be used at all temperatures to be safe. Generally used antiseptics are ethyl acetate, amyl acetate, toluene, formaldehyde, etc.
In a typical autolysis, a slurry or cream of live yeast cells (15-18 weight percent solids) is plasmolyzed with 2-5 weight percent salt (based on cell weight). Ethyl acetate is added as an antiseptic at 1% by volume to prevent bacterial growth. The slurry is heated up to 50.degree.-55.degree. C. and held at that temperature for 12-24 hours, or until the desired degree of solubilization is reached. The resulting autolyzed yeast material is then pasteurized at 80.degree.-100.degree. C., cooled, and centrifuged or filtered. The filtrate may be concentrated to a paste of about 70-80 weight percent solids, or spray dried to give a powdery yeast autolyzate product. Alternatively the autolyzed mixture may be directly spray dried without the removal of the cell debris to give an autolyzed yeast product. These two products are considerably different in their composition, flavor, and utility.
The use of exogenous enzymes to enhance the autolysis of yeasts is a new approach, although various enzymes have been used in a number of different processes. For example, U.S. Pat. No. 3,088,879 to Weaver teaches the use of papain as an initiator for liberating the cell contents of certain fungal cells containing a mycelial structure. In particular, the examples teach treatment of Pencillium chrysogenum and Agaricus campestris (mushroom). The treatment conditions taught are 50.degree.-85.degree. C. and a pH of 4.3-6.3. The reference does not suggest the applicability of papain to other microorganisms such as yeasts, and no mention is made of autolysis.
U.S. Pat. No. 3,523,801 to Shiga et al. teaches a non-autolytic process for preparing seasonings from microbial cells which includes initially subjecting the microbial cells to an alkaline solution for 1-3 hours at 50.degree.-80.degree. C. to destroy the cell walls, followed by neutralization and apparently a pasteurization step by heating at over 100.degree. C. for 10-15 minutes The thus-treated cells are then treated with a suitable protease enzyme to decompose the proteins at 50.degree.-60.degree. C. for about 6-48 hours. This process involves a combined chemical and enzymatic treatment rather than an autolysis process.
U.S. Pat. No. 3,778,513 to Shiga et al. similarly teaches a method for preparing extract seasonings by first decomposing the cell walls with hydrochloric acid at 100.degree.-130.degree. C. for 1-5 hours. After neutralization, the decomposed material is subjected to a protease enzyme which breaks down the remaining protein to form a peptide mixture. Additional components are added to the peptide mixture and the mixture is subsequently heated and reached at 50.degree.-80.degree. C. for 0.5-5 hours to give a seasoning product. As is clear from the initial step, autolysis is not suggested by this reference since the cells would be inactivated by the high temperature acid treatment.
U.S. Pat. No. 3,809,780 to Ishida et al. teaches a method of preparing a seasoning agent wherein yeasts are subjected to specific cell lytic enzymes of the general Coprinus, Daedaleopsis, or Irpex. The yeast cells can be alive, although heat-killed cells are preferred. The cells are subjected to a decomposition by the enzyme at 45.degree.-55.degree. C. for 5-48 hours at a pH which is dependent upon the specific enzyme used. Although the decomposition conditions used in this process would promote autolysis if living cells were used, the reference clearly suggests that heat-killed cells are preferred, as indicated by the fact that all examples teach heat-treating of the cells at about 100.degree. C. or more for about 10 minutes. Autolysis cannot occur after such a treatment because the enzymes would be inactivated. In addition, Ishida et al. teaches only the use of specific enzymes and does not suggest that any other enzymes will perform satisfactorily in such a process. It particularly does not suggest the enzymes of this invention in combination with autolysis.
As indicated, autolysis is a generally time-consuming process with low efficiency and poor quality assurance. In addition, it has been found that typical autolyzates, such as those produced by the use of plasmolyzing agents like sodium chloride, are poor growth stimulants for lactic acid bacteria starter cultures for making cheese. It is therefore an object of this invention to improve the yeast autolysis process, particularly increasing the yield of the yeast autolyzate and shortening the time needed for the completion of digestion. It is a further object of this invention to produce an effective growth stimulant for lactic acid bacteria starter cultures for making cheese.